man/vzctl.8: minor cleanups to "enter --exec" description

(1) Separate "enter" in SYNOPSYS so it is clear that --exec only applies
    to "enter" command.

(2) Expand the "enter" description, refer to ssh(1) as a proper way.

(3) Simplify EXAMPLES a bit.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
1 file changed